Paving aggregate silos--sometimes referred to as surge bins--are employed in connection with large scale paving operations, where paving mix is prepared at a mixing plant and is hauled to the paving site in dump body vehicles. The mixer at the central plant discharges batches of mix from time to time, but it is impracticable to so schedule mixing and vehicle operation that the mixer can discharge each batch of mix into a waiting vehicle. Therefore the freshly mixed material is carried directly from the mixer into a silo, and vehicles are loaded from the silo.
A mixing plant used for large scale paving operations should preferably be portable, so that upon completion of one paving project it can be transported to the site of a new one. Obviously the rather high and bulky silo cannot be moved from place to place while in its upright operating position, and therefore it has to be tilted down to a horizontal position for transport and swung back up to its upright position at the new site. Setting up and taking down the silo involves manipulation not only of the storage bin itself but also of the rather long conveyor-like elevator that carries material up from the mixer to the inlet in the top of the bin.
Self-erecting silos are available that are so arranged as to be transportable on a single chassis which carries both the bin and the elevator. Heretofore, however, such self-erecting silos have been unsatisfactory because they were necessarily of rather light construction and lacked such desired features as heavy duty floor systems, heavy duty elevator chains and heavy bearings. More sturdily built portable silos, which had to be transported on two or more transport units, had to be erected and taken down with the aid of a crane.
This need for a crane was not just an inconvenience but a matter of very substantial expense. The cost of a crane represents too large a capital investment to justify owning it for the infrequent occasions when a mixing plant is to be taken down and set up, and renting a crane can involve complicated problems of scheduling, liability coverage and transport of the crane to and from the plant site, in addition to the high cost of crane rental itself.
What has been needed, therefore, is a portable silo which has the necessary sturdiness and heavy duty construction for long-continued trouble-free operation but which can be set up and taken down without the need for a crane.
An obvious problem involved in devising a self-erecting heavy-duty silo is that of providing a mechanism by which the long, bulky and heavy storage bin can be swung between its horizontal transport orientation and its upright operative position. This problem is complicated by the need for having the silo rest on long, sturdy legs when in its operating condition, so that its bottom end is elevated high enough to permit dump-body vehicles to be driven under it for loading. If the silo is not to be lifted by means of a crane that pulls it to its upright position, the most practical expedient for raising it is a hydraulic cylinder jack mechanism that pushes it up.
A hydraulic cylinder can exert a substantially high thrust through a fairly substantial distance, but commercially available hydraulic cylinder jacks have been regarded as incapable of extension and retraction through the very considerable distances needed for raising and lowering a paving mix silo. Calculations for one contemplated arrangement for a heavy-duty self-erecting silo showed that it would need a hydraulic cylinder with a stroke of about 21 feet. Engineers experienced in the design of hydraulic cylinders pointed out that a unit of such extreme length would be impractical and inadvisable, especially since the cylinder would not be operating in a vertical position and would therefore be subjected to bending loads that would severely deflect it.
Of course any self-erecting silo arrangement must meet certain requirements in addition to the basic capability for being put up and taken down with self-contained mechanism that is transported on the vehicle units that carry the silo assembly. Safety is a paramount consideration, inasmuch as putting up the silo and taking it down involve the movement of very heavy and bulky objects through large distances. The mechanism employed for setting up and taking down the silo must of course be compact so that it can fit onto the carrying vehicle, along with the silo elements themselves, without exceeding load limits that are fixed by law. And while meeting these stringent requirements, the mechanism that is used only for setting up and taking down the silo must be as inexpensive as possible, so that its share of the total cost of the assembly does not represent a large capital investment that stands idle most of the time.